Grip Sizes for Racquetball
Most racquets seem to be supplied with a grip size between three and a half to four inches, with the average being three and five-eighths inches. Many players find that this is too small.
The simplest solution is to add another grip on top of the one supplied with the racquet. This is also often done by both badminton and squash players.
You will probably have to experiment a little until you get it exactly as you want it.